Method for operating a magnetic inductive flowmeter and magnetic inductive flowmeter

Abstract

The invention relates to a method for operating a magnetic-inductive flowmeter, wherein the magnetic-inductive flowmeter has a measuring tube for conducting a flowable medium, at least two measuring electrodes for detecting a measuring voltage induced in the medium based on the flow rate, and a magnetic-field generating device for generating a magnetic field through the measuring tube, said magnetic-field generating device having a coil. The method has the following steps: - applying a control voltage to the coil in order to generate a change in the coil current over time; and - determining the change in the coil current over time in a transition region, wherein the coil current changes in the transition region until a target coil current (I 目标 ) is reached.

Description

BACKGROUND

[0001] Magnetic inductive flowmeters are used to determine the flow rate and the volume flow of a flowing medium in a pipe. The magnetic inductive flowmeter has a magnet system which generates a magnetic field perpendicular to the flow direction of the flowing medium. A single coil is usually used for this purpose. In order to achieve a uniform prevailing magnetic field, pole shoes are additionally formed and attached, so that the magnetic field lines run essentially perpendicular to the lateral axis or parallel to the vertical axis of the measuring tube over the entire tube cross section. A measuring electrode pair attached to the side of the measuring tube taps off an electrical measuring voltage or potential difference which is applied perpendicular to the flow direction and the magnetic field and which occurs when an electrically conductive medium flows in the flow direction while the magnetic field is applied. Because, according to Faraday's induction law, the tapped-off measuring voltage depends on the velocity, the flow rate U of the flowing medium, and by means of the known pipe cross section, the volume flow u can be determined from the induced measuring voltage determining the volume flow u.

[0002] Magnetic inductive flowmeters are often used in process and automation technology for fluids with a conductivity of up to about 5 μS / cm. Corresponding flowmeters are marketed by the applicant in various designs for various fields of application, for example under the name PROMAG.

[0003] Conventional magnetic inductive flowmeters have a housing around the measuring system which is configured to protect the measuring system from damage and external influences, such as external magnetic fields. Without shielding, external magnetic fields influence the magnetic field generated inside the measuring tube, which leads to measurement errors. SUMMARY

[0004] It is an object of the present application to provide a method for operating a magnetic inductive flowmeter with which influences due to external magnetic fields can be detected.

[0005] It is furthermore an object to provide a magnetic inductive flowmeter which is designed to carry out the method.

[0006] This object is achieved by the method for operating a magnetic inductive flowmeter according to claim 1 and by the magnetic inductive flowmeter according to claim 12.

[0007] The method for operating a magnetic inductive flowmeter according to the application,

[0008] The magnetic inductive flowmeter has a measuring tube for conducting a flowable medium, at least two measuring electrodes for detecting a measuring voltage induced in the medium which is dependent on the flow rate, and a magnetic field generating device for generating a magnetic field through the measuring tube,

[0009] wherein the magnetic field generating device has a coil;

[0010] and the method comprises the following method steps:

[0011] - applying a control voltage to the coil in order to generate a change of the coil current over time; and

[0012] - determining a change of the coil current over time in a transition region,

[0013] wherein in the transition region there is a change of the coil current until a target coil current I 目标 .

[0014] It is particularly advantageous that the presence of a disturbing influence can be inferred via the monitoring of the change of the coil current over time. Thus, no additional sensors, such as Hall sensors or probes, which monitor the magnetic field obtained in the measuring tube, are required in or on the measuring tube.

[0015] Possible disturbing influences on the magnetic field obtained in the measuring tube are the external magnetic field of a permanent magnet, fluctuations in the ambient temperature, ferromagnetic components in the conducting medium, damage to the magnetic field generating device, production errors and / or external magnetic fields, for example of an electric motor or generator.

[0016] A conventional magnetic inductive flowmeter has a coil which is operated with direct current with alternating current direction. The time period in which the coil current has reached its target value and the coil current is essentially constant is referred to as the measurement range. The time period between two successive measurement ranges is referred to hereinafter as the transition region. Changing the sign of the applied coil voltage results in a change in the coil current in this time period until it has reached the target coil current I 目标 .

[0017] The target coil current I 目标 can be the coil current on the basis of which the adjustment is based, and when said target coil current is reached, the voltage induced in the flowing medium is measured. In this case, such a target coil current is referred to as the coil measurement current. However, a method for adjusting the coil current is also known with which the coil current in the transition region is adjusted to a further target coil current, namely the peak coil current, which is greater than the coil measurement current and reduces the time which must be waited until the coil current has reached the coil measurement current after the transition.

[0018] Advantageous developments of the application are the subject matter of the dependent claims.

[0019] One development additionally provides that the transition region comprises a gamma time range (γ),

[0020] wherein a change of the coil current over time is determined in the gamma time range (γ),

[0021] wherein the gamma time range (gamma) is followed by a delta time range (delta) in which the change of the coil current has an opposite sign compared to the coil current change of the gamma time range (gamma).

[0022] The gamma time range is characterized by a coil current flowing throughout the entire time period. In other words, a zero crossing is not within the gamma time range. Further, it is advantageous that the coil current value within the gamma range is greater than the coil measurement current or greater than the target value of the coil current. Thus, the gamma time range comprises a part of the coil current curve characterized by a stabilization of the coil current. Advantageously, the coil current values of the gamma time range are taken which lie essentially on a straight line or deviate from this straight line by not more than 1%, in particular not more than 0.5%, and preferably not more than 0.2%.

[0023] The delta time range follows the gamma time range and is characterized by a change of the coil current having an opposite sign compared to the change of the coil current within the gamma time range. The delta time range comprises a part of the coil current curve which is characterized by a stabilization of the coil current until a constant coil current or target value is reached. The delta time range essentially ends when the coil current deviates from the coil measurement current by only 1%.

[0024] It is not possible to take the time range in which the eddy currents are minimal because this only occurs when the coil current is constant, the result of which is that the self-induction cannot be determined. Advantageously, the coil current or the change of the coil current within the gamma time range is measured and used for determining the self-induction because the eddy currents within the time range are stable or constant and their influence on the self-induction is minimal. It is thus possible to distinguish between influences caused by the eddy currents and influences caused by external interference sources.

[0025] An development provides that the eddy currents occurring within the gamma time range are essentially constant over time.

[0026] A further development provides that the gamma time range (gamma) has a gamma time range start (T γ起点 ),

[0027] wherein in the gamma time range start (T γ起点 ) the value of the coil current is at least 75%, in particular at least 85%, and preferably 95% of the value of the target coil current I 目标 .

[0028] It is particularly advantageous that the change of the coil current for determining the self-induction of the coil is not determined until the coil current in the transition region has reached a value of at least 75%, in particular at least 85%, and preferably 95% of the value of the target coil current I 目标 , wherein the target coil current can be the coil measurement current or the peak coil current. The gamma time range end (Tγ终点 ).

[0029] One development provides that the change of the coil current in the epsilon time range is determined,

[0030] wherein the coil current change in the epsilon time range flows in one direction.

[0031] For generating a magnetic field, a control voltage is applied to a coil arrangement comprising at least one coil. The resulting characteristic coil voltage curve has several time ranges. The epsilon time range describes the time period in which the coil current changes its flow direction. It comprises the zero crossing of the coil current without current flowing through the coil and the time periods immediately preceding and immediately following.

[0032] It is particularly advantageous if it is detected that the coil current changes in the epsilon time range and thus also that the coil current is essentially in a state of zero or at least minimum. It is thus possible to minimize the electrical resistance and thus the temperature-dependent influence on the electrical resistance of the coil winding.

[0033] A further development provides that the epsilon time range (epsilon) has an epsilon time range start (T ε起点 ),

[0034] wherein the value of the coil current at the epsilon time range start (T ε起点 ) is less than 15%, in particular less than 10%, and preferably less than 5% of the value of the target coil current I 目标 .

[0035] A further development provides that the epsilon time range (epsilon) has an epsilon time range end (T ε终点 ),

[0036] wherein the value of the coil current at the epsilon time range end (T ε终点 ) is less than 15%, in particular less than 10%, and preferably less than 5% of the value of the target coil current I 目标 .

[0037] One development also provides the following method steps:

[0038] - measuring the coil voltage applied to the coil; and

[0039] - determining the self-induction of the coil by means of the change of the coil current over time and the coil voltage.

[0040] Inductance is a property of a circuit or component, in particular of a coil. A distinction is made between self-induction (also called self-induction) and mutual induction. The self-induction L of a circuit links the rate of change of the current I over time to the voltage U:

[0041] U = L - dI / dt

[0042] Mutual inductance (or inductive coupling) is the mutual magnetic influence of two or more spatially adjacent circuits by electromagnetic induction due to a change in magnetic flux.

[0043] The interference influences the self-induction of the entire system of the magnetic inductive flowmeter. By monitoring the self-induction, it is possible to detect and compensate the interference influence without the need to attach additional sensors in or on the measuring tube. For this reason, the evaluation circuit is designed to determine the self-induction from the coil current or from the change in the coil current and the coil voltage.

[0044] Advantageously, the measuring circuit is configured to determine the coil voltage and the coil current or the change in the coil current.

[0045] A development additionally provides the following method steps:

[0046] - outputting the change in the coil current over time or a quantity dependent on the change in the coil current.

[0047] Advantageously, the change in the coil current or the quantity dependent on the change in the coil current and the properties of the entire system are transmitted to the user by means of the output unit. The user thus receives an additional monitoring parameter, which in particular provides an advantage in applications in which ferromagnetic components are conducted in the medium.

[0048] A development additionally provides the following method steps:

[0049] - determining a correction term assigned to the deviation of the determination of the change in the coil current over time from a reference value, and

[0050] - taking into account the correction term and the measured measurement voltage value U meas and / or calculating a corrected measurement voltage value U corr from the measured measurement voltage value.

[0051] A development provides that the reference value is determined by means of a mathematical model, a calibration method and / or a simulation program,

[0052] wherein the deviation of the change in the coil current from the reference value is assigned a correction term in each case.

[0053] For certain applications, a permanent diagnosis of the magnetic field state is not sufficient. It is precisely in the field of drinking water that it is stipulated that flowmeters are not susceptible to external interference influences such as, for example, caused by permanent magnets.

[0054] It is therefore advantageous if the evaluation circuit is configured to correct the detected deviation of the change in the coil current from a reference value and the corresponding influence or falsification on the determined flow measurement value. The reference value is predetermined during the adjustment of the magnetic inductive flowmeter and is stored in a memory unit of the evaluation circuit.

[0055] The magnetic-inductive flowmeter according to the application is characterized in that the flowmeter has an operating, measuring and / or evaluation circuit which is configured to carry out the method.

[0056] Measuring circuits in the field of flow measurement technology are known. The purpose of the measuring circuits is to detect very small absolute values and changes in the respective measured variable. There are a number of different developments, each with its advantages and disadvantages.

[0057] On the one hand, the measuring circuit can be configured to tap the potential at one of the measuring electrodes relative to a reference potential. Thus, even if one of the two measuring electrodes fails, it is still possible to determine the flow from the determined potential. The housing potential or the ground potential is suitable as the reference potential.

[0058] On the other hand, the measuring circuit can be designed to detect and record the prevailing potential difference between the two measuring electrodes or the two groups of measuring electrodes.

[0059] In addition, the measuring circuit can be configured to determine the coil voltage and the coil current or the change in the coil current at the coil.

[0060] The measuring circuit thus comprises an analog / digital converter which converts the incoming signal, in this case the potential difference currently present at the respective measuring electrode pair, the prevailing potential at the respective measuring electrode or the current flowing through the coil, into digital data, which are then further processed or stored by the evaluation circuit. However, other measuring converters or measuring transducers from the field of digital measurement technology are also known and suitable for detecting the measured voltage or potential.

[0061] The evaluation circuit is configured to process the measured values of the respective measured variable measured by the measuring circuit and to determine the sought measured variable. The evaluation circuit thus usually comprises a microprocessor, an amplifier and a noise filter. The measuring and evaluation circuit can be of modular design and can communicate by means of a wireless connection or can be part of a single electronic measuring and evaluation unit arranged in the flowmeter housing.

[0062] A particular development provides that the coil voltage time curve applied to the coil has a transition region and a measurement range,

[0063] wherein the transition region and the measurement range alternate,

[0064] wherein the control voltage of the measurement range and in particular the control voltage of the transition region directly after the measurement range have opposite signs.

[0065] It is particularly advantageous to wait for the induction measurement voltage of the transition region and to take the measurement only in the measurement range in order to determine the flow measurement value, since the magnet system will have stabilized in this time period. The polarity reversal of the voltage source leads to a stable zero point and makes the measurement insensitive to influences from inhomogeneities in the multiphase substance, the liquid or low conductivity.

[0066] A specific development provides that the change in the coil current is determined in the transition region.

[0067] Preferably, the coil current or the change in the coil current is determined in the transition region, since the coil current in the measurement range is constant or the change in the coil current is minimal.

[0068] A specific development provides that the control voltage is constant in the transition region,

[0069] wherein the control voltage assumes a first voltage value in the transition region,

[0070] wherein the control voltage assumes a second voltage value in the measurement range,

[0071] wherein the first voltage value is greater than the second voltage value.

[0072] This characteristic coil voltage curve minimizes the time for which the coil current is stable. As a result, the measurement range starts earlier and the operating frequency can be increased. BRIEF DESCRIPTION OF DRAWINGS

[0073] The application is explained in more detail with reference to the following figures. The following sections are shown:

[0074] Figure 1 : magnetic inductive flowmeter; and

[0075] Figure 2 : diagram showing the change in coil current and coil voltage over time. DETAILED DESCRIPTION

[0076] The structure and the measurement principle of the magnetic inductive flowmeter 1 are known in principle (see Figure 1). The medium with electrical conductivity is conducted through the measuring tube 2. The measuring tube 2 usually comprises a metal tube or a plastic tube. The magnetic field generating device 4 is installed such that the magnetic field lines are oriented perpendicular to the longitudinal direction defined by the measuring tube axis. A saddle coil or a pole shoe with a coil 5 is preferably suitable as the magnetic field generating device 4. When the magnetic field is applied, a potential distribution is produced in the measuring tube 2 which is tapped by two measuring electrodes 3 which are oppositely installed on the inner wall of the measuring tube 2. Usually, two measuring electrodes 3 are used which are arranged radially and form an electrode axis which extends perpendicular to the magnetic force lines and to the longitudinal axis of the measuring tube 2. From the measured measuring voltage and taking into account the magnetic flux density, the flow rate of the medium can be determined and, taking into account the cross-sectional area of the tube, the volume flow rate can be determined. If the density of the medium is known, it will be possible to determine the mass flow rate. In order to prevent the measuring voltage applied to the first and second measuring electrodes 3 from being dissipated via the tube, the inner wall is lined with an insulating material or a plastic lining.

[0077] The magnetic field established by means of the coil and pole shoe arrangement is generated by a timed direct current with alternating flow direction. The operating circuit 6 is connected to the two coils 5 and is configured to apply a control voltage with a characteristic curve to the coil arrangement with which the coil current is adjusted. The characteristic curve of the control voltage can be seen in Figure 2 . The polarity reversal of the voltage source ensures a stable zero point and makes the measurement insensitive to the influence of multiphase substances, inhomogeneities in the liquid or low electrical conductivity. The measuring and / or evaluation circuit 7 reads the voltage applied to the measuring electrodes 3 and outputs the flow rate of the medium and / or the calculated volume flow rate and / or mass flow rate. In Figure 1 the cross section of the magnetic inductive flowmeter 1 shown, the measuring electrodes 3 are in direct contact with the medium. However, the coupling can also occur in a capacitive manner. According to the invention, the measuring and / or evaluation circuit 7 is also configured to measure the coil voltage actually applied to the coil and the coil current or the change in the coil current over time. A display unit (not shown) outputs the determined change in the coil current over time or a quantity which depends on the change in the coil current. Alternatively, a message or a warning message can be output if they deviate from stored reference values. The reference values are determined by means of a mathematical model, a calibration method and / or a simulation program. However, this is not sufficient, in particular in applications in the field of drinking water. For this purpose, the measuring and / or evaluation circuit 7 is configured to correct the determined deviations by means of stored correction factors.

[0078] Figure 2 A diagram is shown which describes the change in the coil current B and the coil voltage A over time.

[0079] The depicted coil voltage curve A is characterized by two constant voltage values, which are applied to the coil for a certain period of time, wherein the two voltage values and the respectively assigned period of time are different. However, operating methods for magnetic inductive flow meters are also known, in which only a constant voltage value with alternating polarity or a voltage curve is applied, wherein the voltage value is adjusted with respect to time. However, the advantageous time range, in which the influence on the self-induction of the coil is minimal, also applies to the other operating methods. The illustrated curve A shows time intervals which repeat over time and whose sign alternates. In the prior art, voltage curves are known which have a rest phase between the periods of time in which the voltage is applied to the coil, in which the decay behavior of the voltage is determined without the voltage being applied.

[0080] The depicted coil current curve B has two regions. In the first region, the coil current changes due to the change on the applied coil voltage. This region is also referred to as the transition region. In the second region, the coil current and the coil voltage are essentially constant. For this reason, it is therefore generally waited until the coil current has stabilized and is constant or reaches a target value before the measurement voltage is tapped at the measurement electrodes and the flow measurement value is determined. The second region is also referred to as the measurement range. Furthermore, the coil current curve has three further characteristic time periods (ε, δ and γ).

[0081] The ε time range encompasses the period of time in which the coil current changes the flow direction. This is also referred to as zero crossing. Just at the zero crossing, no coil current flows through the coil, and the influence of the resistance of the coil on the self-induction of the coil is negligibly small. This is particularly advantageous, since the influence of the temperature-dependent electrical conductivity of the coil winding material is thereby eliminated. The ε time range has an ε time range start (T ε起点 ) and an ε time range end (T ε终点 ). The ε time range begins when the value of the coil measurement current is less than 15%, in particular less than 10%, and preferably less than 5% of the value of the target coil current I 目标 . The ε time range ends when the value of the coil measurement current is greater than 15%, in particular greater than 10%, and preferably greater than 5% of the value of the target coil current I 目标 , wherein, according to the depicted development, I 目标 is the peak coil current.

[0082] The γ and δ time ranges follow one another and essentially cover the periods of coil current overshoot or stabilization. The δ time range includes the period in which the sign of the coil current change is opposite to that of the γ time range. Furthermore, the change in coil current decreases within the δ time range until the coil current has reached the target value and the second region begins. The γ time range includes the period immediately preceding the δ time range; however, in this range, the coil current flows through the entire time range. That is, the zero-crossing of the coil current does not occur within the γ time range.

[0083] Preferably, the coil current values ​​in the γ-time range are characterized in that they are greater than the target value of the coil current in the second region or measurement range. Furthermore, the γ-time range is selected such that the coil current values ​​lie on a straight line. It has been found that eddy currents exhibit a stable state within the γ-time range and do not change or only change slightly. Therefore, their influence on the coil self-inductance is also minimal. As a result, even the smallest deviation of the self-inductance relative to the reference value due to external influences can be detected and compensated. When the value of the measured coil current is the target coil current I... 目标 The value of γ is at least 75%, particularly at least 85%, and preferably 95%, when the starting point of the γ time range (T) is... γ起点 (Starting from the beginning, where, according to the described development, the target coil current I) 目标 Corresponding to the peak coil current. γ time range end (T) γ终点 The self-inductance is defined by the starting point of the δ time range, specifically by the time at which the coil current changes direction. Preferably, the measurements used to determine the self-inductance are derived only from the γ time range.

[0084] List of reference numerals

[0085] 1. Magnetic induction flow meter

[0086] 2 Measuring tube

[0087] 3. Measuring electrodes

[0088] 4. Magnetic field generating equipment

[0089] 5 coils

[0090] 6. Operating Circuit

[0091] 7. Measurement and / or evaluation circuits

[0092] ε ε time range

[0093] γ time range

[0094] δ δ time range

[0095] I 目标 Target coil current

[0096] T γ起点 γ time range start point

[0097] T γ终点 γ time range end point

[0098] T ε起点 ε time range start point

[0099] T ε终点 ε time range end point

Claims

1. A method for operating a magnetic induction flowmeter (1), in, The magnetic induction flowmeter (1) comprises: a measuring tube (2) for conducting a flowable medium; at least two measuring electrodes (3) for detecting a flow velocity-related measuring voltage induced in the medium; and a magnetic field generating device (4) for generating a magnetic field passing through the measuring tube (2). The magnetic field generating device (4) includes a coil (5); The method includes the following steps: - Apply a control voltage to the coil (5) to generate a change in the coil current over time; and - Determine the change in the coil current over time in the transition region. During the transition region, the coil current changes until the target coil current I is reached. 目标 , The conversion region includes the γ time range (γ), Specifically, the change of the coil current over time within the time range (γ) is determined. Wherein, the γ time range (γ) is immediately followed by the δ time range (δ), wherein the change in the coil current has the opposite sign to the change in the coil current within the γ time range (γ). - Measure the coil voltage applied to the coil (5); and - The self-inductance of the coil (5) is determined by means of the change of the coil current over time and the coil voltage.

2. The method according to claim 1, in, The eddy currents that appear in the time range (γ) are essentially constant with time.

3. The method according to claim 2, in, The γ time range (γ) has a starting point (T) of the γ time range. γ起点 ), Wherein, at the starting point of the γ time range (T) γ起点 In the above, the value of the coil current is the target coil current I. 目标 The value is at least 75%.

4. The method according to claim 3, in, At the start of the γ time range (T) γ起点 In the above, the value of the coil current is the target coil current I. 目标 The value is at least 85%.

5. The method according to claim 4, in, At the start of the γ time range (T) γ起点 In the above, the value of the coil current is the target coil current I. 目标 95% of the value.

6. The method according to any one of claims 1-5, comprising the following method steps: - Output the change in the coil current over time or the amount that depends on the change in the coil current.

7. The method according to any one of claims 1-6, comprising the following method steps: - Determine a correction term, which is assigned to a determined deviation of the change in coil current over time relative to a reference value, and -Considering the aforementioned correction term and the measured voltage value U meas And / or calculate the corrected measured voltage value U based on the correction amount depending on the measured measured voltage value. corr .

8. The method according to claim 7, in, The reference value is determined using mathematical models, calibration methods, and / or simulation procedures. In each case, the deviation of the change in the coil current over time relative to the reference value is assigned a correction term.

9. A magnetic induction flowmeter, characterized in that... The flow meter has operating, measuring, and / or evaluating circuitry (6, 7) configured to perform the method according to any one of claims 1-8.

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